Self explanatory.
If you don't get it, let me explain for yourself. "
Buttering the bean", or "
buttering your bean", is in essence busting your nut full of semen on a girls clitoral region, aka the "bean".
"I'm gonna butter your bean you
dirty whore"
Guy 1 - "Oh brah, did you butter her bean
last night?"
Guy 2 - "
Yeah dude I hope I didnt get her preggers"
Girl - "MMMMMMM he buttered my bean so good last night, it took us hours to clean it all up!"